INTRODUCTION: Medication nonadherence is a prevalent issue among patients with bipolar disorder, leading to substantial negative consequences. Despite documented cognitive deficits in this population, the relationship between executive dysfunction and medication nonadherence remains unclear. This study aims to investigate the association between executive functions and medication adherence in euthymic patients with bipolar I disorder. METHOD: In this cross-sectional, comparative study, we recruited 200 euthymic bipolar I disorder patients aged 18 to 55 years from the outpatient clinic of Iran Psychiatric Hospital in Tehran in 2024, using a convenience sampling method. The euthymic phase was confirmed using the Persian versions of the Young Mania Rating Scale and the Hamilton Rating Scale for Depression. Patients completed the Medication Adherence Rating Scale, along with a series of executive function tests including Go/No-Go, Wisconsin Card Sorting Test, and Iowa Gambling Task. Multivariate analysis of covariance was employed to analyze the results, controlling for demographic and clinical variables as covariates. RESULTS: Of the participants, 54.5% had low medication adherence. Low adherent patients exhibited significantly poorer performance in Go/No-Go as indicated by higher commission errors (F [1] = 7.63, p = 0.006) as well as the Wisconsin Card Sorting Test, evidenced by a higher number of perseveration errors (F [1] = 8.61, p = 0.004) and fewer completed categories (F [1] = 6.67, p = 0.011), compared to high adherent patients. Notably, although differences in decision-making were observed between the two groups, these did not reach statistical significance (p = 0.139). CONCLUSIONS: This study establishes a correlation between low medication adherence and deficits in executive functions-specifically response inhibition and cognitive flexibility-in patients with bipolar I disorder. Furthermore, even after controlling for covariates, the differences in executive functions between medication adherence groups remained significant.